Tamoxifen therapy has great potential for the treatment of tumors of the female reproductive tissues. Unfortunately, many of the tumors are resistant to the therapy and grow hormone-independently. The overall objective of the proposed studies is to elucidate the molecular mechanism of estrogen action in the tumor cells with the hope that a better understanding about action of estrogens and anti-estrogens can be translated clinically into the development of a more effective anti- estrogen therapeutic model. The effects of estrogen and anti-estrogens are mediated through the estrogen receptor (ER). Multiple studies have shown that the ER can be activated by non-steroid growth signals such as peptide growth factors. The cross talk between growth factors receptors and the ER may stimulate the hormone-independent growth of ER-positive tumors and contribute to the resistance of these tumors to tamoxifen. Our preliminary indicate that the human ER is activated in tumor cells by MEKK1, which may be involved in mediating this cross talk. The specific aims of the proposed studies are to elucidate the molecular mechanism underlying the activation, to further characterize the specificity of the activation, and to examine the functional relationship between the activation and hormone-independent tumor growth and tamoxifen resistance. To achieve these objectives, the signaling steps from MEKK1 to the ER will be dissected by biochemical approaches involving in analyzing receptor phosphorylation and assaying the receptor activity in the presence or absence of either activity or dominant negative forms of kinases known to be activated by MEKK1. The cell specificity of the ER activation in tamoxifen-sensitive and hormone independent ovarian, breast and uterine tumor cells will be analyzed and compared. Finally, a stable and inducible expression of a dominant negative form of MEKK1 will be established in the tumor cells to test whether the inhibition of endogenous MEKK1 activity blocks the Hormone-independent growth of the tumor cells and changes their sensitivity to tamoxifen.